Variations of the EW of the Hα line. Phase 0.0 corresponds to the time of the first optical spectrum of the August 2013 campaign (HJD 2 456 507.7607). The solid curve shows the expected variations as reconstructed from the amplitudes and phases of the ν₂ and 2 × ν₂ frequencies (see Fig. 5), whilst the data points with error bars correspond to the actual EWs measured in August 2013.